This invention relates to frequency converters in general more particularly to an improved frequency converter and method of operating same.
One known type of frequency converter comprises an inverter with controlled converter valves in a bridge circuit, the output terminals of which are connected to a parallel resonant circuit and the input terminals of which are connected to an a-c voltage source, via an intermediate d-c link having a smoothing choke and a controlled rectifier with the input terminals of the inverter bridged by an auxiliary commutating arrangement which contains thyristors in a bridge circuit and having capacitor connected across its the a-c outputs. A method for operating, and in particular, starting up such a frequency converter includes, during the start-up phase, firing thyristors of corresponding diagonal bridge arms of the inverter and of the auxiliary commutating device prior to every zero crossing of the resonant circuit voltage and, after the conclusion of the start-up phase, firing only thyristors of diagonal bridge arms of the inverter while the thyristors of the auxiliary commutating arrangement remain unfired.
Such a frequency converter and such an operating procedure are known from the German Offenlegungsschrift No. 2,056,847. In the steady state condition, frequency converters having parallel resonant circuit as the load are load controlled and the operating frequency of the inverter is determined by the resonance frequency of the resonant circuit. The current passes in direct commutation from one bridge arm of the inverter to the next one that carriers current, the reactive commutation power being supplied by the capacitor of the parallel resonant circuit. In the start-up phase, the charge of the capacitor is not sufficient for direct commutation and indirect commutation is necessary during this time. In the know frequency converter, an auxiliary commutating arrangement is provided for this purpose. This auxiliary commutating arrangement consists of a capacitor which can be replenished by a d-c voltage source and which is connected in series with the d-c terminals of the bridge circuit of thyristors. The a-c terminals of the bridge circuit are connected via a choke and a further capacitor in series. In the known frequency converter, there is further provided as an initial current arrangement a thyristor and a capacitor in series, the latter being shunted by a resistor, and which likewise bridges the input terminals of the inverter. For starting up, the thyristor of the initial current arrangement is fired in the known frequency converter until a current of predetermined magnitude flows in the smoothing choke. When this current is reached, thyristors of diagonal bridge arms of the inverter are fired; current now flows through the load and excites an oscillation, and the thyristor of the initial current arrangement is extinguished. Shortly before every zero crossing of the resonant circuit voltage, the previously non-conducting thyristors of the inverter and thyristors in the corresponding diagonal bridge arms of the auxiliary commutating arrangement are subsequently fired. The voltage of the rechargeable capacitor of the auxiliary commutating arrangement now is present as a negative cup-off voltage at the thyristors of the inverter and extinguishes the thyristors which had heretofore been conducting. Now, the voltage at the capacitor of the bridge circuit of the auxiliary commutating arrangement rises until the fired thyristors of the inverter can take over the current, whereupon the current carrying thyristors of the auxiliary commutating arrangement are extinguished. A choke which is connected in series with this capacitor is required for this voltage rise at the capacitor of the bridge circuit. This switching cycle is repeated shortly before every following zero crossing of the resonant circuit voltage. In the process, the capacitor of the bridge circuit of the auxiliary commutating arrangement is charged more and more and its voltage counteracts the voltage of the rechargeable capacitor, so that the auxiliary commutating arrangement becomes ineffective after a certain period of time, whereby the start-up phase is concluded, since now the resonant circuit voltage of the inverter is sufficient for direct commutation and the frequency converter operates in the normal operating condition. The known frequency converter is not load controlled during the start-up phase but is self-commutating with the frequency of the resonant circuit voltage. However, this advantage is obtained at great expense. In particular, an additoional power supply is required for charging the capacitor in the auxiliary commutating arrangement.
Thus, there is a need to simplify a frequency converter of the type mentioned above and at the same time to improve its operation during the start-up phase.